1. Field of the Invention
The present invention generally relates to an automatic transmission having a planetary gear mechanism and an engagement mechanism.
2. Description of Related Art
In the past, there is a known automatic transmission, having: a planetary gear mechanism, having a plurality of elements rotatable inside a casing; and a plurality of engagement mechanisms, switchable to a coupled state in which the elements are coupled to one another, or switchable to a fixed state in which the elements are fixed to the casing.
It is known that in such automatic transmission, as the engagement mechanism, an engagement mechanism allowing rotation (normal rotation) of the elements of the planetary gear in a prescribed direction and preventing the rotation (reverse rotation) in a direction opposite to the prescribed direction is used (for example, a one-way clutch)(for example referring to patent document 1).
Besides, in recent years, due to the demand of increasing gear positions, an automatic transmission using the following switching mechanism (for example, a two-way clutch) as the engagement mechanism is being developed. The switching mechanism is switchable between a reverse rotation preventing state allowing the normal rotation of elements of the planetary gear mechanism and preventing the reverse rotation and the fixed state. In such switching mechanism, switching is performed by a hydraulic pressure control circuit, etc.